Dr. Zachary Uriah "Zack" Addy is a professional forensic anthropologist who worked for the Jeffersonian Institute. He was originally Dr. Brennan's brilliant young assistant at the beginning of the series before he received his doctorate in Forensic Anthropology in the series' second season. In the third season, Dr. Addy was discovered to have been secretly under the influence of the psychotic serial killer Gormogon and aiding him from within the Jeffersonian. He is currently serving time in a state psychiatric hospital for his part in Gormogon's crimes.

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Coming from a large Michigan family (with three brothers and four sisters), Zack is a former child prodigy, a genius with an IQ well above 163 and a supposed photographic memory. He has started two doctorates, one in Forensic Anthropology, which he has completed, and one in Engineering, which was mentioned in "The Woman in the Car."

His specialty, like Dr. Brennan, is in the analysis of remains, especially identifying cause of death and weapons from marks remaining on skeletal remains. It is usually his task to remove the flesh from the bones, a process known as debriding. Because of his tremendous intellect, he has a strong broad-based knowledge of many of the specialties in the Jeffersonian lab.
Despite his intelligence, Zack is unsure of himself and though he has come up with crucial insights vital to some of the team's cases, he is unable to forcefully express his opinion to Dr. Brennan. This may be due to hero worship and also the feelings he might have toward her. When he discovered that Temperance's own Forensic Anthropology professor had become her lover, Zack repeatedly wondered aloud whether he might enter into a similar relationship with Dr. Brennan. He was quickly disabused of the notion by his colleagues.
In The Wannabe in the Weeds episode of Bones, it was discovered that Zack was a singer during his childhood as a way for his parents to help integrate him socially. Though it did not appear so, this fact becomes evident in a scene with Hodgins as a way to prove himself when Hodgins doubted him. According to Emily Deschanel, Zack "almost definitely has Asperger syndrome," a high-functioning brand of autism.[1]

The episode The Perfect Pieces in the Purple Pond reveals Zack is receiving psychological treatment from Sweets. It is also shown at least Hodgins and Angela have both been in contact with Zack during his imprisonment. Hodgins is shown bringing Zack a mathematical riddle to solve. In his session with Sweets, Zack admits to feeling guilty for killing the lobbyist, but claims he was not actually crazy, arguing he committed the crime for what had appeared to be perfectly logical reasons at the time. "I was wrong, not delusional," he says. Sweets believes Zack should feel more guilty about killing the lobbyist and less about having been taken in by Gormogon's rhetoric.

Toward the end of the episode, Zack escapes from the institution. It is revealed he could have escaped at any time but did not feel it was necessary to do so until that particular moment. After helping his former coworkers solve the case, Zack willingly returns to the institution with Sweets and Booth.

After Booth relinquishes Zack to Sweets' hands, Zack tells Sweets that while he had helped Gormogon find the lobbyist, he himself had not actually stabbed him. In other words, Zack believes he killed the lobbyist, but from a legal standpoint he is considered only an accessory or co-conspirator. Sweets insists Zack change his story, but Zack refuses. Zack fears if his secret were to come out, he will find himself in prison, where he is sure to do very poorly according to Jack Hodgins. He reminds Sweets as Zack's therapist, he must not reveal Zack's secret because if Sweets were to do so, he would be violating doctor-patient confidentiality. The episode closes with Zack behind bars and Sweets keeping Zack's secret.

In the season 4 finale, Zack is considered a suspect when a man is murdered in a popular nightclub owned by Booth and Dr. Brennan. Zack is described as a "moron who goes to jail for a murder he didn't commit" by Vincent, suggesting Booth (at least subconsciously) knows Zack is innocent. At the end of the episode, we learn the entire thing was a dream Booth had while in a coma and part of a new book Brennan was writing.

Zack also returns as a guest star in the season 5 episode The Parts in the Sum of the Whole. We see him in a flashback as Dr. Brennan's graduate student while she works on her first case with Booth.

Despite his intelligence, Zack is unsure of himself and though he has come up with crucial insights vital to some of the team's cases, he is unable to forcefully express his opinion to Dr. Brennan. This may be due to a hero worship and also to romantic feelings toward her. When he discovered Temperance's own Forensic Anthropology professor had become her lover, Zack repeatedly wondered aloud whether he might enter into a similar relationship with Dr. Brennan. He was quickly disabused of the notion by his colleagues.[2]

Zack appears to have an on and off-again relationship with "Naomi in Paleontology," despite hints in the first season she was dissatisfied with his sexual prowess[3]. In the third season, they accompany each other to the annual Jeffersonian Institute Halloween party, agreeing to dress as the front and back half of a cow ("Mummy in the Maze"). Zack has no problem making inappropriate comments about others' personal lives, and asked Agent Booth for advice on sex and women, requests which Booth characteristically ignored. Angela answered his question once, and Hodgins gave him a book (Kama Sutra) to help him out ("The Pain in the Heart").

According to Emily Deschanel, Zack "almost definitely has Asperger's syndrome," a condition on the autism spectrum.[4] Although well-meaning, helpful, and friendly, when a situation calls for social interaction or intuition, he is often lost. Further evidence of his social ineptitude can be seen in the frequent, on-screen coaching in social matters he gets from Jack and Angela. He is very literal and is often confused by colloquial expressions or metaphor, despite his high intelligence. His attempts to use such expressions meet with mixed success, such as referring to a skull he'd cleaned as being "clean enough to eat off of." ("Two Bodies in the Lab")

In the episode "The Killer in the Concrete", Booth is on the phone with Zack and Dr. Brennan while looking for "Icepick" (real name, Hugh Kennedy) at a model airplane enthusiasts gathering. Unaware of Zack's interest in planes, Booth comments every "airplane freak" in the area was at the event, and Zack corrects him by saying the enthusiasts prefer to be called "pilots." Zack mentions forensic anthropology is only one of his doctorates, the other is in applied engineering, and he is extremely adept to practical aeronautics.

As seen in the episode "The Man in the Fallout Shelter" Zack has a large family and greatly values them, remarking the true meaning of Christmas is "Brothers, sisters, nieces, nephews; 40 people who love you and are happy to see you." Also in the first season, he reveals to Dr. Brennan he uses his vacation time to visit his family. While he does not necessarily enjoy these visits, he goes because they love him ("The Man in the Wall").

Episodes towards the end of the first season of Bones reveal that Zack's colleagues, especially Dr. Goodman, feel he has become too comfortable as Dr. Brennan's assistant and is therefore not completing any of his Doctorates to avoid having to grow into a new position. Goodman and Hodgins conspire to make Zack less comfortable in his position to motivate him to complete his studies and assume a role above that of an assistant.

Right before he was about to complete his doctorate, Zack asks Dr. Saroyan if he could have a job working at the Jeffersonian, but she replies that she could not put him in front of a court to testify because people would not take him seriously. Zack then goes to ask Angela for fashion advice and she completely redoes his look including a new haircut and suit. After completing his doctorate, and getting a makeover from Angela, Zack again asks Dr. Camille Saroyan for a job, and she gives it to him.

At the end of the second season, Zack receives a request from the office of the President to ship out to Iraq. What his duties there would be is not revealed, and he only tells Hodgins and Booth about it. He initially seems inclined to go, but after talking about the reality of war with Booth, he seems less sure, asking Booth for advice because he "knows more about duty and honor than anyone else" he knows. He also turns down the offer to be best man at Hodgins and Angela's wedding in case he decides to go to Iraq and is killed, because he doesn't want Hodgins' memories of the wedding to be tainted with sadness. In the first episode of season 3, it is revealed that he did in fact go to Iraq but returned in that same episode.

In the final episode of season three, The Pain in the Heart, Zack receives third-degree burns and massive tissue damage (the cartilage on his left hand was destroyed, among other things) on both hands after an explosion in the lab. It is later revealed that he is working as the apprentice of the The Gormogon serial killer, and the explosion was intended as a distraction so Gormogon could break into the lab and steal a skeleton. Zack's weaker personality was manipulated by the Gormogon (whom he referred to as "the Master") so completely that Zack believed his belief system irrefutable, even killing a lobbyist and dividing his bones among the Jeffersonian's storage units. However, he still maintained a loyalty to his friends, willing to injure himself to keep Hodgins safe

Zack in the hospital in the final episode of season 3 with Dr. Brennan

and giving up the location of Gormogon's house after Brennan refuted his logic.

At the end of the episode, the prosecutor Caroline Julian says that while Zack has confessed to killing the lobbyist she is willing to work out a deal to have him declared non compos mentis, a decision which will commit him to an asylum rather than prison. Zack will no longer be a regular character on the show, but series creator Hart Hanson said that he may become a recurring character to provide consults to the team with "certain talents we can use in a 'Hannibal Lecter' kind of way." [5]

Zack begins to receive psychological treatment from Sweets (The Perfect Pieces in the Purple Pond). Feeling guilt about aiding a serial killer and admitting to his mistake, Sweets believes that Zack shouldn't feel bad about being wrong, but about being delusional during the period of being Gormogon's apprentice. Not only that, Sweets believes that Zack should actually feel guilty about killing the lobbyist more than about being wrong.

Zack later escapes the institution by swapping the magnetic strips from Sweets' key card to Zack's library card. After solving the case, Zack realizes it's time to go back to the institution where Sweets was waiting. After Booth reliquishes Zack to Sweets' hands, Zack admits to not killing the lobbyist and that the previous appentice had killed the lobbyist instead. Since the Gormogon can only have one apprentice at a time, he killed the original lobbyist's murderer in order to take Zack under his wing.

Though not the actual killer, Zack does not want the truth to be revealed to his colleagues at the Jeffersonian because if his secret were to come out, Zack would find himself in prison since he is still an "accessory" to the lobbyist's murder. Out of fear of finding himself in prison (due to Hodgins' firm belief that Zack "would NOT do well in prison"), he reminds Sweets that, as Zack's therapist, he must not reveal Zack's secret. If Sweets were to do so, he'd be violating doctor-patient confidentiality.

In the finale The End in the Beginning he plays the part of Brennan's assistant at the nightclub, which reflects the situation that he and Brennan had in the previous seasons.

Zack appears to have an on-again, off-again relationship with "Naomi in Paleontology", despite hints in the first season that she was dissatisfied with his sexual prowess. In the third season, they accompany each other to the annual Jeffersonian Institute Halloween party, agreeing to dress as the front and back half of a cow.

Zack seems to be friends with Jack Hodgins, with whom it was once thought he was roommates. He actually lives in the apartment above Hodgins garage on the grounds of his large estate. In more than one episode, Hodgins refers to Zack as his "best friend." He often competes with Hodgins to be "King of the lab", a title claimed by the one who makes a pivotal discovery or conclusion. In Season 1's episode, The Man in the Bear, Zack competes with Hodgins over a beautiful delivery lady. In fact, he rents from and also carpools with Hodgins, since he can't drive or ride a bike, simply because he refuses to learn. He once made a comment to Booth that if he (Booth) had the same understanding of structural engineering, he would be afraid to drive as well.